Genesis of petrichor

Abstract

“Petrichor”, well known to mineralogists as argillaceous odour, is commonly observed as the pleasant and refreshing odour which frequently accompanies the first rains after a warm dry period. Several possible mechanisms have been considered in connection with the origin of this odour. These include the synthesis of odorous compounds on the clay or rock surface by spontaneous catalysis of atmospheric gases, the sorption of organic compounds from the atmosphere, catalytic transformation of sorbed compounds and microbial activity. Evidence is presented which suggests the atmosphere contains, as general contaminants, lipids, terpenes, carotenoids and other volatile decomposition products from animal and vegetable matter. The sorption of these compounds, or their oxygenated derivatives, by rocks and clays is controlled by the properties of the sorbent and the partial water vapour pressure of the atmosphere, low relative humidities favouring maximum uptake. Oxidation and transformation of sorbates take place on the rock surface and are accelerated by warm to hot climatic conditions. The odorous and volatile products of these processes are subsequently displaced from the pores of the rock by moisture when the relative humidity of the atmosphere approaches saturation. The possibility of a relationship between petrichor and petroleum formation is discussed.